Electrically powered outdoor illumination devices are widely used to illuminate pathways, yards, parks and other like areas. Commonly, such illumination devices are connected to public utility systems, or similar sources of electrical power and are controlled by preset timing devices, to illuminate desired areas at nightfall and automatically turn off at a predetermined time, for example, prior to daybreak.
Many traditional illumination devices require extensive cabling, suitable timing mechanisms and the like, and are thus relatively expensive to install and maintain. Moreover, such illumination devices utilize electric power generated in a conventional manner such as by burning fuel. Burning fuel contributes to contamination of the environment and depletion of existing fuel resources.
More recently, self-contained solar powered illumination devices which utilize photovoltaic devices to charge batteries which, in turn, activate a light source contained therein, in the absence of sunlight, have been used for illumination and/or decorative purposes. Such self-contained devices have limited battery power and thus, typically utilize low wattage bulbs, particularly incandescent bulbs which do not generate sufficient light to provide clear illumination in the areas desired. Use of incandescent bulbs provide a low level of light and render such self-contained illumination devices particularly impractical for security applications or the like. Alternatively, if sufficient illumination is provided, the battery power is insufficient to maintain the illumination for the time desired.
Fluorescent lamps are widely used to provide illumination in traditional electrically powered illumination devices used for general lighting purposes because they are more efficient than incandescent bulbs in generating light. A fluorescent lamp is a low-pressure gas discharge source, in which light is produced predominantly by fluorescent powders activated by ultraviolet energy generated by a mercury plasma forming an arc. The lamp, usually in the form of a tubular bulb with an electrode sealed into each end, contains mercury vapor at low pressure with a small amount of inert gas for starting. The inner walls of the bulb are coated with fluorescent powders commonly called phosphors. When the proper voltage is applied, the plasma (forming an arc) is produced by current flowing between the electrodes through the mercury vapor. This discharge generates some visible radiation. The ultraviolet in turn excites the phosphors to emit light.
Two electrodes are hermetically sealed into the bulb, one at each end. These electrodes are designed for operation as either "cold" or "hot" cathodes or electrodes, more correctly called glow or arc modes of discharge operation. Electrodes for glow or cold cathode operation may consist of closed-end metal cylinders, generally coated on the inside with an emissive material. Conventional cold cathode lamps operate at a current order of a few hundred milliamperes, with a high cathode fall or voltage drop, something in excess of 50 volts.
The arc mode or hot cathode electrode is generally constructed from a tungsten wire or a tungsten wire around which another very fine tungsten wire has been uniformly wound. The larger tungsten wire is coiled producing a triple coil electrode. When the fine wire is absent, the electrode is referred to as a coiled-coil electrode. This coiled-coil or triple-coiled tungsten wire is coated with a mixture of alkaline earth oxides to enhance electron emission. During lamp operation, the coil and coating reach temperatures of about 1100.degree. C. where the coil/coating combination thermally emits large quantities of electrons at a low cathode fall of the order of 10 to 12 volts. The normal operating current of hot cathode lamps presently ranges upwards to 1.5 amperes. As a consequence of the lower cathode fall associated with the "hot" cathode, more efficient lamp operation is obtained and, therefore, most fluorescent lamps are designed for "hot" cathode operation.
The lamp life of hot cathode lamps is determined by the rate of loss of the electron emissive coating on the electrodes. Some of the coating is eroded from filaments each time the lamp is started. Also, during lamp operation evaporation of emissive material occurs. Although electrodes are designed to minimize both of these effects, the end of the lamp life is reached when either the coating is completely removed from one or both electrodes or the remaining coating becomes non-emissive. Because some of the emissive coating is lost from the electrodes during each start, the frequency of starting hot cathode lamps influences their life. The rated average life of hot cathode fluorescent lamps is usually based on three hours of operation per start.
Cold cathode lamps on the contrary are not appreciably affected by starting frequency because of the type of electrode used. Cold cathode fluorescent lamps emit light in the same way as do standard hot electrode lamps. These operate as normal glow discharges and their electrodes are uncoated hollow cylinders of nickel or iron. The cathode fall is high and to obtain high efficacy or power for general lighting purposes, conventional lamps are made fairly long, about 3 m, with a diameter of about 20 mm or 25 mm. About 2000 V is required for starting these conventional lamps, and about 900 V to 1000 V for running.
The advantages of cold electrode lamps compared with the hot electrode lamps are that they have a very long life, usually 15000 hours or more, in consequence of their rugged electrodes and low current consumption. They start immediately, even under cold ambient conditions. Their life is unaffected by the number of starts. Also, they may be dimmed to very low levels of light output.
Some self-contained lamps have utilized hot cathode fluorescent bulbs in an effort to provide increased illumination. Such lamps have used a simple circuit including a transformer with a single transistor to generate a square wave. However, this has resulted in a substantially decreased lamp life and high current consumption. A square wave degenerates the characteristics of the hot cathode fluorescent bulb. In addition, a major disadvantage of using hot cathode fluorescent bulbs in self-contained lamps lies in their inability to function properly at low ambient temperatures.